Optical filters can be advantageously combined with various technologies. One such area is telecommunications. Telecommunication optical filters typically require minimal insertion loss and polarization dependent loss (PDL). Various polarization diversity techniques have been employed to achieve minimal insertion loss and PDL for birefringent filters.
Polarization diversity techniques split the light input into the optical filter into its orthogonal components for filtering. The filtered light may then be recombined and output as an unpolarized beam.
Some prior art polarization diversity techniques have used cube polarization beam splitters to split the input light and to recombine the filtered light. Unfortunately, cube polarization beam splitters have large beam spacing, must be precisely aligned, and do not always provide high extinction ratios.
Other prior art polarization diversity techniques have used walk-off configurations with expensive bulk birefringent materials such as calcite or yttrium vannadate. The walk-off configurations require complex optical paths when using both outputs in a polarization insensitive manner. This results in an expensive and complex architecture.
Polarization mode dispersion (PMD) has become increasingly important as optical data rates have increased. Complex architectures have additional delay symmetry elements to compensate for PMD. Simpler architecture can also compensate for PMD. The simpler architecture requires the use of extra compensating elements of precise thickness. Unfortunately, the asymmetric architecture of these PMD compensated devices renders them sensitive to temperature changes.
Accordingly, there is a strong need for an inexpensive optical filter with a small beam spacing and simple optical path that has little or negligible PMD.